CORS error with Ionic/Angular application and PHP API

Question

Access to Xmlhttprequest at 'https://exemplodedominio.com/app-api/api/login-register.php' from origin 'http://localhost:8100' has been blocked by CORS policy: Response to preflight request doesn’t pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested Resource.

I know this question is highly repeated here on the site, but none of the many other same questions had an answer that solved my problem.

I’m using a PHP API for an application made in Ionic and Angular, but the above error occurs not only when running the application with the non-local API link, but also when opening the application (already compiled) on a mobile device.

Access-Control-Allow-Orign is already released with * in the PHP header, as shown in the API headers below:

  header("Access-Control-Allow-Origin: *");
  header("Access-Control-Allow-Credentials: true");
  header("Access-Control-Allow-Methods: POST, GET, OPTIONS");
  header("Access-Control-Allow-Headers: Content-Type, Authorization, X-Requested-With");
  header("Content-type: application/json; charset=utf-8");
  header("Content-type: text/plain; charset=utf-8");

My previous application is this:

    server: string = "https://exemplodedominio.com/app-api/api/";

    postData(body, file){
        let type = "application/json; charset=UTF-8";
        let headers = new Headers({ 'Content-Type': type });
        let options = new RequestOptions({ headers: headers });

        return this.http.post(this.server + file, JSON.stringify(body), options)
        .map(res => res.json());
    }

• exemplodedomain.com is a fictional domain. What I use is another.

(I rule out the possibility of replacing with proxies, so my last option)